1. Field of the Invention
The present invention relates to signal processing in a wireless communication network and particularly to signal processing in this network when multiple chains are used.
2. Related Art
In accordance with the guidelines promulgated in IEEE 802.11b, a packet can include a preamble followed by a header. All 802.11b-compliant systems support a long preamble, which includes 128 bits for a synchronization (Sync) field and 16 bits for a start frame delimiter (SFD) field. A short preamble, which includes 56 bits for the Sync field and 16 bits for the SFD field, is provided to improve throughput when data such as voice, VOIP, or streaming video is being transmitted.
Before transmission, the preamble (whether long or short) is spread using a Barker code that has eleven “chips”, i.e. 10110111000. Note that after the preamble, other parts of the packet can be spread using a Barker code or a CCK code, depending on the data rate used for transmission.
Upon receiving a signal, a receiver can use cross-correlation to try and match the Barker code with the received signal. Notably, a Barker code has certain mathematical qualities that can help the receiver detect the packet and determine the boundaries of each symbol, thereby providing synchronization. After synchronization, the receiver can begin further processing of the packet, e.g. decoding and other processing.
Typically in decoding, a rake receiver can be used to counter the adverse effects of multipath. That is, in wireless communication, radio signals can reach a receiver's antenna by multiple paths (created because of, for example, reflection off of physical objects). Referring to FIG. 1, because each signal travels at the speed of light but travels by a different path to the receiver's antenna, a receiver can receive the multipath signals 101 as energy pulses over a period of time.
A rake receiver typically includes a plurality of sub-receivers (called fingers herein), wherein each finger has a small delay that allows tuning to an individual multipath component. FIG. 2 illustrates an exemplary finger 201 that collects symbol energy for a rake receiver 200. In this embodiment, a matching block 207 can receive an input RF signal and assign each finger of rake receiver 200 an energy “peak”. The assigned peak can be provided to a Barker correlator 202, a delay equalizer 204, and code generators 205. Barker correlator 202, with additional input from code generators 205 (which generate the code for identifying symbols at the assigned energy) and the received RF signal, can provide de-spreading of the received RF signal for both I and Q branches. A phase rotator 203 can receive outputs of Barker correlator 202 and a channel estimator 206 (which also receives outputs of correlator 202) to estimate the phase-corrected state of the received RF signal channel. Delay equalizer 204, which also receives the outputs of phase rotator 203, can determine the appropriate compensation of the delay for the difference in arrival times of the symbols in finger 201. This output can then be provided to combiner 208, which receives outputs from other fingers (which can include identical function blocks to finger 201 and therefore are shown merely as additional arrows pointing to combiner 208).
Thus, as noted in FIG. 2, although each multipath component can be decoded independently, the rake receiver subsequently combines the multipath components in combiner 208. Notably, in a multipath environment, a direct signal is not necessarily the best signal. Thus, by combining the multipath components, an optimized signal can be obtained, thereby improving receiver performance.
In one embodiment called an equal-gain combining, the rake receiver outputs can be combined by weighting each output equally. In another embodiment called maximal ratio combining (MRC), each output is weighted to maximize the signal-to-noise ratio (SNR) of the combined output.
As proposed, IEEE 802.11n is a standard directed to handling multiple chains (facilitated by MIMO, i.e. multiple-input multiple-output), in contrast to the single chain described above in accordance with 802.11b. An existing technique for handling multiple chains is to use switch diversity, e.g. determine which chain provides the highest SNR and selecting the signals from that chain for decoding. However, combining signals from all chains could provide better receiver performance. Therefore, a need arises for improving the processing of signals in the context of IEEE 802.11n, i.e. wherein the wireless network includes multiple chains.